1. Field of the Invention
This invention relates generally to the dispensing of material under pressure and relates particularly to a piston used with a barrier pressure container in which material to be dispensed is located on one side of the piston and fluid under pressure is disposed on the opposite side of the piston.
2. Description of the Prior Art
Heretofore many efforts have been made to dispense material from relatively small containers by means of pressurized fluids. Generally these prior art methods have fallen into three categories. The first category has been the aerosol type in which a liquid or gas is mixed with a hydrocarbon such as freon or the like having a low boiling point. When the valve of the container is operated, the pressure within the container is relieved so that some of the hydrocarbon boils off and causes the gas or liquid within the container to be sprayed through the discharge opening.
The second category of dispensers has been of the barrier pressure type in which the container is divided into first and second compartments by a movable piston. The material to be dispensed is disposed within one of the compartments and the other compartment is provided with a gas, such as air, under pressure. In this type of construction, when the dispensing valve is open, the gas under pressure moves the piston against the material to be dispensed and causes such material to be discharged through the valve.
The third category includes a container having a flexible collapsible bag therein and such bag receives the material to be dispensed while the area within the container and exteriorly of the bag is filled with fluid under pressure. A dispensing valve communicates with the interior of the bag so that when the dispensing valve is open the fluid under pressure within the container squeezes inwardly on the bag and causes material to be dispensed.
The present invention is concerned only with the second category of dispensers in which the container is provided with a movable piston for dispensing material.
In the past, it has been difficult to fill barrier pressure containers with material to be dispensed without trapping air or other gas within the container. The entrapment of air has caused many problems, particularly when dispensing viscous materials such as caulking compound and the like from a barrier pressure container, since the material is introduced into the container while the container is in a non-pressurized condition. Any air which is trapped within the material compartment of the container during the filling process becomes pressurized when the area on the opposite side of the piston is pressurized since the piston compacts such material. If the pressurized air in the material compartment is located at the top adjacent to the discharge nozzle, such air is discharged as soon as the valve is operated and causes material to spatter over a wide area. Any air trapped adjacent to the piston will be compressed and normally such compressed air escapes through the discharge valve when most of the contents of the container have been dispensed and causes material being dispensed to spatter.
Any air trapped in the material may have a deleterious effect on both material and container, particularly if the material and container are sensitive to oxygen. As an example, caulking compound begins to cure when subjected to oxygen in the air and such curing makes the dispensing of the product more difficult. Air trapped in the container and exposed to the inner wall of such container may cause oxidation, particularly in the area of the side seam.
In order to dispense substantially all of the material from the container, many pistons have been provided with an upper surface having a configuration generally corresponding to the configuration of the inner surface of the top of the container. Since the dispensing valve normally extends into the container, these pistons have been provided with a central recess to accomodate the inner portion of the dispensing valve.
When material to be dispensed is being introduced into the containers, the filling operation can be either a top filling or a bottom filling operation. In a top filling operation the bottom wall of the container and the piston are placed within the container and the container is arranged along a generally vertical axis so that a filling tube can be inserted into the container substantially to the upper surface of the piston and thereafter the filling tube is withdrawn as material is being discharged into the container. The rate of withdrawal of the filling tube normally is complementary to the flow of material into the container. After a predetermined quantity of material has been discharged into the container, the filling tube is removed and a top wall, normally having a discharge valve therein, is attached to the container in any desired manner, as by crimping, welding or the like. When the piston has a central recess or cup for receiving the inner portion of the dispensing valve, air has frequently been trapped within such cup during the filling operation and such air has been compressed when the area below the piston was subsequently pressurized.
In the bottom filling of a container, the top wall of the container is applied and the container is inverted so that the top wall and the valve are located at the bottom, while the body of the container is disposed along a generally vertical axis. Thereafter a filling tube is inserted through the open bottom of the container and material is discharged into the container as the filling tube is withdrawn. When the container has been substantially two-thirds filled, the introduction of material is interrupted and the filling tube is removed. Thereafter the piston is pushed into the container until the piston engages the material to be dispensed. During this movement air trapped between the piston and the material is easily forced past the side walls of the piston since the area behind the piston has not been pressurized. However, air trapped within the central recess or cup is sealed therein by the material as soon as the piston engages such material and therefore the air trapped within the cup cannot escape.